1. Field of the Invention
A method for measuring the principle axis of stress patterns, particularly those due to vertical fracturing, of earth layers beneath a body of deep water. The azimuthal alignment of a horizontal borehole is optimized based on the result of such measurements.
2. Discussion of Related Art
Although the art of seismic exploration is very well known, it will be briefly reviewed to provide definitions of technical terms to be referenced herein.
An acoustic source of any desired type such as, by way of example but not by way of limitation, a vibrator, an explosive charge, a sonic boom, an air or gas gun, or an earth impactor, is triggered to propagate a wavefield radially from the source location. The wavefield insonifies subsurface earth formations whence it is reflected therefrom to return to the surface. The mechanical earth motions due to the reflected wavefield are detected as electrical signals by an array of seismic receivers or receiver groups distributed at preselected spaced-apart group intervals, at or near the surface of the earth, along a designated line of survey, offset from the source.
Hereafter for brevity, the term "receiver" unless otherwise qualified, means either a single seismic receiver or a relatively compact group of interconnected seismic receivers. The mechanical motions detected by the receivers are converted to electrical or optical signals which are transmitted over ethereal, electrical or optical data-transmission links to a multi-channel recording device. Usually, each receiver is coupled to a dedicated recording channel. An array may encompass many tens or hundreds of receivers which are coupled by a transmission link to a corresponding number of data-recording channels. To reduce the need for an excessive number of individual data transmission lines between the receivers and the recording channels, the receivers share a relatively few common transmission lines and the signals from each receiver are multiplexed into the appropriate data-recording channels by any convenient well-known means.
In operation, the selected source type successively occupies a plurality of source locations along the line of survey, emitting a wavefield at each location. After each emission, the source is advanced along the line by a multiple of the receiver spacing interval. At the same time, the receiver array is advanced along the line of survey by a corresponding spacing. In other arrangements such as for use with 3-D studies, the sources and receivers are emplaced at the intersections of a uniformly-spaced coordinate grid and the data are binned.
Sedimentary earth layers are initially laid down more or less horizontally. Later, the earth layers may become tilted or stressed so that vertical fracturing occurs, particularly in the more brittle rocks such as, for example, the Austin chalk. If the orientation of the principal axis of stress is known, horizontal boreholes can be directed perpendicular to the fracture plane to maximize oil and/or gas recovery. Seismic methods may be used in geophysical exploration to study vertical fracturing.
Because of anisotropy, it is known that the velocity of seismic waves, including both compressional waves and polarized shear waves, propagating through the earth layers varies as a function of the azimuth of the wavefield trajectory relative to the orientation of the plane of a vertical fracture pattern. The propagation velocity of a seismic wavefield is faster parallel to the fracture plane, that is, along strike, than perpendicular thereto. For various reasons, use of shear waves is preferred over compressional waves by geophysicists because there is said to be a greater velocity contrast. For example, see U.S. Pat. No. 4,817,061, issued Mar. 28, 1989 to R. M. Alford et al.
Leon Thomsen, in a paper published in Geophysics, v. 53, n.5, March, 1988, entitled REFLECTION SEISMOLOGY OVER AZIMUTHALLY ANISOTROPIC MEDIA, teaches a method using shear waves. He, like Alford denigrates the use of compressional waves for anisotropy studies because the azimuthal-dependent velocity variation of compressional waves is said not to be very large and is said to be difficult to evaluate particularly with noisy data signals.
Although use of shear waves is preferred, shear waves cannot propagate in a fluid. Therefore, in deep water there is no choice but to use compressional waves. Because shear waves are undetectable directly in deep water, Mallick, in U.S. patent application Ser. No. 08/254,306, filed 06/06/94 and assigned to the assignee of this invention, proposes to use compressional waves in deep water. In his method, the compressional-wave amplitude vectors as derived from CMP (common mid-point) gathers along two or more lines of profile having known but different orientations, are resolved to identify the azimuth of the principal anisotropic axis.
The method of Mallick is a useful approach but it is complex and expensive of data-processing time. There is a need for a deep water system that is easy to implement in the field and that will provide seismic data signals that can be resolved inexpensively in the laboratory.